Method of preparing corrosion resistant metallic articles

ABSTRACT

A method of furnishing the surface of ferroalloys with resistance to hot corrosive gases containing halogens or compounds thereof or to general oxidation which comprises the steps of heating in an oxidizing atmosphere at a temperature of 1,000* to 1,400* C for a suitable number of hours articles prepared from a metal consisting of 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.1 to 0.6 percent of titanium, 0.01 to 0.5 percent of zirconium, iron as the remainder and further containing ordinary impurities, in which constitution ratio of Ti% + Zr% to N% ranges from 10 to 60, so as to diffuse by migration in the surface layer of the base body of the article part of oxidizable elements contained therein, thereby forming an oxide layer mainly consisting of Alpha Al2O3 on said surface.

I United States Patent [15] 3,660,1 73 Matsuno et al. [4 1 May 2, 1972 54] METHOD OF PREPARING CORROSION 2,442,223 5/1948 Uhlig 148/635 RESISTANT METALLIC ARTICLES 2,635,164 4/1953 Rehnquist et al. ..75/l24 X I 2,987,394 6/1961 Mueller et al ..75/124 [72] Inventors: Akira Matsuno; Takao Sasame; Ikiizo 3,068,094 12/1962 Zackay et al ..75/126 D Shimizu; Humio Kizu; l-liroyuki Kagawa,

all of Hiroshima-ken, Japan Primary Examiner-Ralph S. Kendall Attorney-Flynn & Frishauf [73] Assignees: Toyo Kogyo Co., Ltd., Hiroshima-ken;

Tokyo Shibaura Electric Co., Ltd., [57] ABSTRACT Kawasaki-shi, Japan i A method of furnishing the surface of ferroalloys with re- Filed: June 23,1970 sistance to hot corrosive gases containing halogens or com- [211 Appl. NOJ 49,052 pounds thereof or to general oxidation which comprises the steps of heating in an oxidizing atmosphere at a temperature of 1,000 to l,400 C for a suitable number of hours articles [30] Foreign Application Priority Data prepared from a metal consisting of 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent June 25, 1969 Japan ..44/50588 f carbon, to 105 percent of nitrogen to 0 p cent of titanium, 0.01 to 0.5 percent of zirconium, iron as the [52] U.S. Cl ..l48/6.35, 75/124, 75/ 126, remainder and f h Containing ordinary impurities, in 143/315 which constitution ratio of Ti% 21% to N% ranges from 10 [51] Int. Cl. ..C23f 7/04 to 60, so as to difiuse by migration in the surface layer of the [58] Field of Search... ....l48/6.35, 6.3; 75/126 D, 126 F, base body of the article part of oxidizable elements contained 75/124 therein, thereby forming an oxide layer mainly consisting of llAigOg on saidsurface. [56] References Cited 6 Claims, 7 Drawing Figures UNITED STATES PATENTS 2,191,790 2/1940 Franks ..75/124 All203'TiO2 PATENTEDMM 2 [972 FIG. 1

INCREASE 0F WEIGHT BY ox|DAT|0N(m /cm SHEET 1 [1F 3 dAQ 0 +ZrO [(1 MY 2 I972 SHEET 2 CF 3 0 O O O guzwmmm o NEE 5'0 HEATING HOURS The present invention relates to a method of preparing metallic articles which are resistant to corrosion, particularly that exerted by hot corrosive gases containing halogens or compounds thereof and general oxidation.

In recent years, public nuisance caused by exhaust gases from automobiles raises a very serious problem, so that there are now studied various measures for its resolution. Among these measures, that which attracts general attention as most effective consists in conducting exhaust gases from the automobile engine, together with air into a reaction vessel to ignite the unburned components of the exhaust gas with or without a catalyst so as to render the noxious ingredients of the exhaust gas harmless. With respect to such method and apparatus therefor, there have already been advanced numerous proposals. Since, however, there has not been available any material adapted for use in said reaction vessel which is fully capable of resisting corrosion caused by exhaust gases from automobiles, the aforesaid method, or any proposal associated therewith has not been put to practical use to date.

As is-well known, exhaust gases from automobiles contain halogens and compounds thereof such as Cl Br PbCl PbBr C H CI and C l-I Br other lead compounds and phosphur compounds in addition to unburned carbon monoxide and hydrocarbons. Since these ingredients are extremely corrosive to general metallic materials, members involved in the exhaust system of an automobile, for example, a muffler is subject to damage by corrosion and consequently deemed as a consumable article. Where there is fitted to an automobile a reaction vessel according to the above-mentioned afterburner method, said vessel, if made of conventional materials, would fail to withstand the strong corrosive action of gases burning therein at a temperature of at least 900 C and sometimes rising to l200 C.

Although ferroalloys containing nickel, chromium and cobalt are generally known to withstand heat, they are unadapted for long use under such severe conditions as prevailing in the aforementioned reaction vessel. Further, even the chromizing or aluminizing treatment of the surface of these heat-resistant ferroalloys does not display any prominent effect. On the other hand, there is proposed a method oflining or spray coating a material of ceramic with very corrosion-resistant ceramics to improve the resistance of the reaction vessel to corrosion and oxidation. However, cohesion between the particles of ceramics themselves and also between said .ceramic particles and the base material of the reaction vessel is so weak that ceramics are liable to come off the base material due to the flow of exhaust gas, repeated heating and the shaking of an automobile during its run, making such reaction vessel also incapable of long use. Although there are used in the field of space development metal materials having prominent resistance to oxidation and heat, yet such materials are too expensive for economical application in general metallic articles.

The present invention has been accomplished with the view of settling the aforesaid problems associated with the material of a reaction vessel for treating or detoxicating exhaust gases from automobiles. However, it will be apparent that the method ofthe present invention is not limited to use in detoxicating automobile exhaust gases, but generally applicable in preparing metallic articles which are required fully to withstand long application in an atmosphere where there prevail very corrosive hot gases such as those containing halogens or compounds thereof. The term metallic articles, as used herein, include raw materials obtained by rolling or drawing, intermediate products worked by pressing, welding or cutting and end articles.

An object of the present invention is to provide ferroalloy articles which are prominently resistant to corrosion caused by very corrosive hot gases containing halogens or compounds thereof and general oxidation.

Another object of the present invention is to form a compact and firm corrosionand oxidation-resistant oxide layers mainly consisting of cit-A1 0 in close contact with the surface of a metallic base body, and enable it to maintain a stable state free from the possibility of coming off even when it is subjected to repeated heating under exposure to corrosive gases introduced at high temperature and speed.

The above-mentioned objects have been attained by preparing articles from a metallic material consisting of 10 to 25 7c of Cr, 2 to 5 ofAl, less than 0.04 ofC, 0.005 to 0.05 of N, 0.1 to 0.6 ofTi, 0.01 to 0.5 ofZr, Fe as the remainder and containing ordinary impurities (the ratio of Ti and Zr to N ranges from 10 to 60), and heating the article for a suitable number of hours in an oxidizing atmosphere at l,000 to l,400 C for oxidation of its surface, and diffusing by migration on the surface of the base body of the article part of oxidizable elements contained therein, thereby forming said surface of an oxide layer mainly consisting of a-Al O The features of the invention which are believed to be novel are set forth with particularity in the appended claims.

The invention itself, however, as to its organization together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. I is a schematic longitudinal sectional view of an apparatus used in testing the degree of corrosion occurring in a ferroalloy article;

FIG. 2 is a curve diagram indicating the relationship of the ratio of (Ti Zr)/N associated with the components of a ferroalloy article versus increased weight due to oxidation;

FIG. 3 is a curve diagram showing the relationship of'the aforesaid ratio of (Ti Zr )/N versus the percentage removal of an oxide layer;

FIG. 4 is a longitudinal sectional view of an oxide layer obtained by the method of the present invention, illustrating a typical structure thereof;

FIG. 5 is a longitudinal sectional view of an oxide layer obtained by the method of the invention, illustrating another typical structure thereof;

FIG. 6 is a curve diagram presenting the relationship of heating time and increased weight due to oxidation according to Example 3; and

FIG. 7 is a plotted diagram from all Examples according to the contents of Ti and Zr, showing the different areas of two oxide layer types.

Heretofore, metallic materials have been discussed with their corrosions distinguished between high temperature dry corrosion, that is, oxidation, and low temperature wet corro sion, namely, erosion, and there have been developed anti-oxidation and anti-erosion alloys according to the applications in which they are used. Among anti-oxidation alloys, that of a Fe-Cr-Al system for electrical heating is most excellent, some forms of this alloy being capable of withstanding high temperatures up to 1,350 C. On the other hand, alloys of a Ni-Cr system or a Fe-Cr system rich in Cr are considered to display relatively high resistance to corrosion by hydrocarbons or halogens and useful insofar as they are used at temperatures up to 300 C.

Corrosion resistance tests on metallic materials are generally conducted by an apparatus illustrated in FIG. 1 wherein there are placed a sample 3 and a substance 4 evolving corrosive gas in a quartz tube 2 kept at a constant temperature by a heater 1 and, while they are continuously heated, there is introduced into the quartz tube 2 air at the rate of 200 c.c. per minute in the direction of the indicated arrow. With this apparatus, there were made experiments to determine how far the aforesaid metal alloys of the prior art could withstand the action of hot corrosive gases. Noting that among Aluminum (Al) is also indispensable to render metallic particles resistant to high temperature oxidation. lf aluminum is contained in less than 2 percent, there will result an insufficient growth of aAl O in an oxide layer, leading to the increased formation of such oxides, for example, (Fe, C 0 as will exhibit less resistance to oxidation and erosion. Beyond 5 percent of aluminum, there will occur decrease in the room temperature workability of metalsas is the case with chromi urn. Accordingly, the suitable range of aluminum content is between 2 and 5 percent, the most preferable content being about 3 percent. As in the prior art Fe-Cr-Al alloys for electri- TABLE 1..COMPOSI'IIONS OF SAMPLES AND CONDITIONS OF EROSION TEST Composition (percent by weight) Condition of erosion Kind of alloy Cr Al Fe Y C Si Mn Others Remainder test Fe-Cr-Al. 20.1 3.0 Trace 0.1 0.1 Ti, 0.3 Fe andimpurities Fo-Cr-Al-Y 20.0 3.0 0.6 Trace 0.4 Trace do r All 900 C., Inconel600 15.8 7.2 0.04 0.2 Trace Ni and impurities Zhours. High Cr castingiron 32.0 0. 04 0.3 Trace }Fe and impurities"...

1 Commercial name of alloys manufactured by International Nickel Company (INCO).

2 Rare earth metal.

With the view ofimproving the erosion resistance of the surface of metallic materials by forming an oxide layer by common practice, there were heated in the air 2 to 50 hours at a temperature of 1,300 C Fe-Cr-Al and Fe-Cr-Al-Y alloys which were known to display relatively good oxidation resistance among those listed in Table 1 above so as to determine the erosion resistance of these systems. The test shows that some samples of the former system were not substantially eroded, proving their strongresistance to erosion and oxidation, Whereas. all the samples of the latter system were prominently affected just like those not subjected to heat treatment.

Analysis of those of the samples which presented an appreciable resistance to erosion and oxidation proves that such samples contained minor amounts of titanium and thattheir surface coated with a firm, oxide layer mainly consisting of aAI O We conducted further study by incorporating in Fe-Cr-Al alloys both Ti and Zr at the same time which formerly were individually added in very minute amounts to prevent the coarse growth of crystal particles of said system at elevated temperature, so adjusting the total amounts of Ti and Zr as to fall within a certain range with respect to the N content, and heat treating the alloy thus prepared for considerably long hours, and as a result discovered that articles manufactured from this alloy prominently increased in resistance to erosion and oxidation. Namely, the method of the present invention is characterized by preparing articles from a metallic material consisting of 10 to 25 of Cr, 2 to 5 of Al, less than 0.04 of C,

- 0.005 to 0.05 of N, 0.1 to 0.6 of Ti, 0.01 to 0.5 of Zr,

Fe as the remainder and further containing ordinary impurities (the ratio of Ti% Zr% to N% ranges from 10 to 60), heating the article for a suitable number of hours in an oxidizing atmosphere at l,O00 to l,400 C for oxidation of its surface to diffuse by migration to the surface of the base body of the article part of oxidizable elements contained therein, thereby fonning on said surface an oxide layer mainly consisting ofaAl O Here is given the reason why the composition of the metallic base body is limited to within the aforementioned range.

Chromium (Cr) is an indispensable component for imparting an oxidationrresistant quality to metallic articles; However, its content of less than 10 percent will be insufficient to provide a desired degree of oxidation resistance. If the content exceeds 25 percent, there will be presented difficulties in rolling, drawing or press work. Accordingly, it is preferred that the content be ranged from 10 to 25 percent, the most desirable amount being around percent.

cal heating, chromium and aluminum are indispensable not only for formation of an oxide layer but also for elevation of the fatigue strength and oxidation resistance of a metallic base body when it is repeatedly heated at high temperature. Amounts of these materials incorporated in the metallic articles prepared by the method of the present invention are much the'same as in these of the prior art.

The carbon (C) component of the metallic base body combines with the content of titanium (Ti), zirconium (Zr) or chromium (Cr) to form carbides, which do not constitute solid solutions in the base body. Generation of large amounts of such carbides will not only lead to the occurrence of hot corrosion, but also to the reduced effect of titanium and zircocium which are added to increase the mechanical strength of an oxide layer. Accordingly, it is preferred that the carbon content be as little as possible, its allowable limit being 0.04 percent.

Nitrogen (N) contained in the metallic base body suppresses the diffusion of carbon and presents the coarse growth of carbide particles. Since, however, nitrogen has greater affinity with titanium'and zirconium than carbon, it will combine with them to form TiN and ZrN, far more decreasing the effect of titanium and zirconium than does the carbon. Therefore, it is desired that nitrogen be contained in the base body in as small amounts as possible, its permissible limit being 0.05 percent. Further, nitrogen largely affects the properties of the oxide layer as described later, according to the ratio of Ti Zr to N. From this point of view, the lower limit of the nitrogen content is set at 0.005 percent.

When the metallic base body is heated to high temperatures in an oxidizing atmosphere, titanium and zirconium serve by their diffusion to cause an unremovable oxide layer to be formed compact and firm on the surface of the metallic base body. Titanium is dissolved in the solid aAl O to render it compact. However, if the content of titanium falls to below 0.1 percent, it will not display a noticeable effect, whereas its content of more than 0.6 percent will reduce the room temperature workability of metals, Accordingly, it is advisable that the titanium content be ranged from 0.1 to 0.6 percent. Even within said range, the properties of a resultant oxide layer appreciably vary with the titanium content as described later. Zirconium fixes nitrogen to promote the effect of titanium, and aids in causing the oxide layer to be deeply settled in the metallic base body, taking a rooty form, thereby reducing its possibility of coming off the base body. From the standpoint of fixing nitrogen, the zirconium content should be at least 0.01 percent. Since, however, the zirconium content of more than 0.5 percent greatly decreases the oxidation resistance of a resultant oxide layer, the preferable range of said content is between 0.01 and 0.5 percent. Even within this range, the properties of the oxide layer vary, as described later, with the zirconium content in relation to the titanium content.

Already known is the incorporation of titanium and zirconium in Fe'Cr-Al alloys. However, the object of it exclusively consisted in preventing the coarse growth of metal particles at high temperature to prolong the life of electrical-heating alloys. Although the alloys of the present invention have a similar secondary effect to that exhibited by titanium and zirconium which are incorporated in a metallic base body in the form of solid solution or form of compounds, there has not been in existence any alloy prepared by positive addition of these elements as in the case of the invention.

Metallic silicon (Si) is generally used as a deoxidant in melting metals, and in addition is incorporated together with zirconium in the form of Fe-Si-Zr alloy in the present case, so that Si is to exist in the base metal of the invention as an impurity. Further, metallic manganese (Mn) is added to fix both the deoxidant and sulfur contained in metals so as to eliminate the harmful effect of them. However, silicon and manganese will not affect the present invention, provided that they are used in customary proportions of less than 0.5 percent and 0.6 percent respectively.

One of the important factors of the present invention is that the value of the ratio (Ti% Zr%)/N% is considered as a parameter common to the increased weight in heat treatment and the peel resistance of the oxide layer. If the ratio decreases from 10, there will result the reduced peel resistance of the oxide layer and if the ratio exceeds 60, the layer will be degraded in oxidation resistance and peel resistance. Accordingly, it is necessary to limit the ratio to within the range of from to 60, the most preferable ratio lying between and 50. FIG. 2 represents curves showing the relationship of the ratio (Ti% Zr%)/N% versus the increased weight due to oxidation of metal samples having a typical composition embodying the present invention which were heat-treated in an oxidizing atmosphere at a temperature of 1,300 C for different lengths of time. Numerals 5, 6 and 7 represent samples heat-treated for 50, 100 and 200 hours respectively. FIG. 3 gives curves illustrating the relationship of the ratio (Ti% Zr%)/N% versus the peel resistance of oxide layers deposited on the surface of the samples subjected to heat-treatment under the similar condition as in the preceding case. The rate of peeling is expressed in the percentage decrease in weight of the aforementioned heat-treated samples which were subjected 5 minutes to sand blasting generally used in surface grinding. Numerals 8 and 9 denote the samples heat-treated 50 and 100 hours respectively.

As seen from FIGS. 2 and 3, the ratio (Ti% Zr%)/N% and the properties of the resultant oxide layer have interrelationships. At the present moment, the reason has not yet been clearly defined. However, it may be considered that the oxidation resistance of an oxide layer decreases with the increasing amounts of titanium and zirconium capable of diffusion, insofar as they do not form compounds of TiN and ZrN respectively, and contrary the peel resistance of the oxide layer is elevated. Said peel resistances would reach maximums when the ratio (Ti% Zr%)/N% stands at 40 to 50, but decrease when the ratio exceeds 60, or when the ratio falls to below 10. Accordingly, we have found that with the ratio (Ti% Zr%)/N% limited to within the range of from 10 to 60 there can be reliably manufactured metallic articles protected with an oxide layer having a desired resistance to erosion and peelmg.

Unless the heat-treatment of a metallic base body is not conducted in the air or an oxygen-bearing atmosphere at temperatures of more than 1,000 C, there will not be formed a desired oxide layer due to the oxidation resisting property of Fe-Cr-Al alloys in itself. However, if the temperature rises above l,400 C, the metallic base body itself will be deteriorated. Accordingly, it is advisable that the heating temperature be ranged between l,000 and 1,400'fC, the most preferable level being around l,300 C. In this connection, it will be noted that heating time is largely governed by an oxygen potential in the atmosphere in which heat-treatment is performed. Where heating is carried out in the air, it is desired to last for at least 2 hours, and may also be extended to 200 hours according to the required thickness of an oxide layer. Upon completion of heating, the metal article in the furnace is allowed to cool in the air.

The oxide layer deposited on the surface of a metallic base body consists of a main component of aAl,O having minor amounts of titanium and zirconium dissolved therein in solids and some mixed crystals of aAl o with some proportions of ZrO (Fe,Cr) O and other oxides. The layer itself is compact and firm and tightly adheres to the metallic base body. It has so strong a resistance to peeling that even where ground with sand paper, it does not readily come off. Moreover, the oxide layer displays a prominent resistance to erosion by halogens, Pb or its compounds and vanadium, which characterizes the properties of metallic articles obtained by the method of the present invention. Accordingly, the aforesaid reaction vessel for treating exhaust gases from automobiles which is coated inside and outside with an oxide layer according to the present invention is not affected at all by halogens, compounds thereof, Pb, Pb compounds or P compounds. Referring to the general oxidation resistance of the metallic article, it only slightly increases in weight by oxidation where it is employed at lower temperatures than those at which the oxide layer is formed, always presenting excellent oxidation resistance.

If the content of zirconium involved in the aforementioned composition of a metallic base body prepared by the method of the present invention is selected to fall within a narrow range of 0.01 to 0.2 percent, then the oxide layer deposited on said base body will assume such a structure as shown in FIG. 4. This structure will hereinafter be referred to as Type A. Type A represents an oxide layer mainly consisting of aAl O and minor amounts of ZrO the outermost region of said layer being constituted by a thin film of Al O TiO This type of oxide layer is not deeply settled in the metallic base body taking a rooty form, displaying a somewhat weaker resistance to peeling than the later described Type B. Since, however, the oxide layer of Type A does not readily come off by sand paper grinding, it is not likely to raise any practical difficulties. Type A mainly results from the diffusing behavior of titanium and is formed under the conditions where the metallic base body contains large amounts of titanium dissolved in solids and minor volumes of TiN as an intervening substance, namely, where the greater part of the zirconium only serves to fix nitrogen.

Formation of a Type A oxide layer is supposed to proceed by the following mechanism. The alloy elements involved in the metallic base body are diffused at a velocity of decreasing order as Al Ti Zr, where there prevails a sufficientoxygen potential for diffusion of aluminum and titanium, aluminum is first diffused at the initial stage of diffusion to form aAl O Since the affinity of alloy elements with oxygen progressively decreases in the order of Zr Ti Al Cr Fe, the titanium which starts diffusion after aluminum is dissolved in aAl O in solids only to a small extent. Most of the titanium passes through the stratum of aAl Q and is diffused up to the outermost region to produce a thin film of aAl O 'TiO which has an extremely compact orthorhombic crystal structure. Once said thin film is formed on the surface of the oxide layer, it obstructs the further diffusion of titanium and aluminum and the intrusion of oxygen ion into the interior of the oxide layer, resulting in a sharp decline in the oxidation of said layer, a departure from the parabolic law constituting a rate-determining factor at the stage where the aAl O is initially formed. (The aforementioned condition is illustrated in FIG. 6 associated with Example 3 given later.) Small amounts of zirconium which do not produce a compound of ZrN in the metallic base body, but are simply dispersed therein without forming solid solutions, begin to be diffused following titanium and pass through the stratum of aAl O However, the zirconium is obstructed in diffusion by the stratum of the aforesaid Al O -TiO 2 and dissolved in solids in the stratum of atAl O disposed near the boundary defined by said ozAl O -TiO system, partly generating a compound of ZrO in this region. (Refer to Example 9 given later.)

If the proportionsof titanium and zirconium involved in the aforementioned composition of a metallic base body according to the present invention are selected to fall within the v ranges of 0.2 to 0.6 percent and more than 0.2 to 0.5 percent respectively and the weight ratio of titanium to zirconium is set'at a larger value than 0.9, then the oxide layer formed on the surface ofa metallic base body according to the method of the present invention will assume such a structure as illustrated in FIG. 5. Said structure will hereinafter be designated as Type B. This type of oxide layer entirely consists of mixed crystals of aAl O and small amounts of ZrO and is deeply set in the metallic base body taking a rooty form, and the outermost part of the layer being constituted of a-Al O mixed with a small amount of ZrO .and (Fe,Cr) O crystals. Accordingly, the layer of Type B presents a very strong resistance to peeling, and its surface withstands far severer conditions than Type A with respect to attacks by halogens or PhD. This layer is slightly less resistant to general oxidation than Type A, but only to such extent that there will not be presented any practical difficulties.

Some samples of the oxide layers of Types A and B prepared according to Examples 3 and 8 respectively were heated 50 hours at 1,300 C and the others 100 hours at the same temperature. All the samples were subjected to sand blasting for minutes to compare their readiness for peeling, the results being presented in Table 2 below. The table shows that Type Apresents less weight increase due to oxidation, but a somewhat larger rate of peeling, whereas Type B displayed a .smaller rate of peeling but greater weight increase due to oxidation. Thus Type B exhibits an extremely strong resistance to peeling, so that even if it is sand blasted, it does not readily come off, except for its outermost surface.

S A AND B OXIDE LAYERS therein. At the same time, oxygen ions having an extremely strong affinity with zirconium are attracted by the diffusing zirconium to be also diffused from the surface to the interior of the metallic base body, generating tetragonal crystals of 210 everywhere in the oxide layer. Further, the oxygen ions go deeper into the base body and react with the zirconium contained therein to form monoclinic crystals of ZrO At this time, excess amounts of oxygen ion produce aAl O near the intergranular area and in consequence the aAl O is supposed to be deeply settled in the metallic base body taking a rooty form as shown in FIG. 5. In the meantime, the titanium which was initially diffused forms appreciable amounts of solid solution in the outermost region of aAl O layer. At the stage, however, where ZrO is growing, titanium is restricted in action by zirconium and does not display any prominent behavior. Yet titanium is dissolved in solids in the aAl O to render the oxide layer compact and firm. FIG. 5 illustrates the structure of a Type B oxide layer wherein the aAl O occupies the greater part of every region, (Fe,Cr)-,,O;, and ZrO- being present in minor weights. Further, ZrO does not combine with the aAl O to form stable crystals, being simply mixed therewith in the form of separate crystals.

It will be apparent that the structures of Types A and B typically illustrated in FIGS. 4 and 5 respectively can not be rigidly distinguished from each other, but depending on the composition of metals involved, there may occur an oxide layer having an intermediate form between Types A and B.

In applying of this invention, the metallic base body to be used may be obtained by melting it in the atmosphere with a conventional method. The oxide layer in this invention is easily formed on the surface of the article by heat-treating in a suitable furnace. For example, in the caseof a large article it may be treated in the state of raw'material, and in the case of small article it may be treated after working. Further, the Type A oxide layer can be forcibly removed by sand blasting, so that 1 The rate of peeling includes the removed amounts of n metallic base body itself, indicating that. till the oxide layer cznne off.

Type B of oxide layer is grown mainly by diffusion of zirconium, on to a surface layer and that of oxygen ion in to a metallic base body. Said growth is realized where there are dispersed in the metallic base body'relatively large amounts of zirconium without generating a compound of ZrN. Generally, increasing addition of zirconium leads to the decreased oxidation resistance of metals, so that to obtain Type B of oxide layer it is necessary that the ratio of titanium to zirconium be set at 0.9 minimum. This will minimize decrease in the oxidation resistance of the metal and at the same time elevate its resistance to peeling and erosion. If the ratio of titanium to zirconium falls to below 0.9 the resistance ofthe metal to general oxidation will sharply drop.

Formation of a Type B oxide layer may be assumed to proceed by the undermentioned mechanism, though not clearly understood. At the initial stage of oxidation, aluminum is first diffused in the surface of a metallic base body as in Type A to form a stratum of aAl O Some amounts of oxidized Fe and Cr crystallize with the stratum of the aAl O in the form of (Fe,Cr) O;,. Titanium is next diffused and then free zirconium dispersed in a metallic base body begins to be diffused through the loose body forward its surface, because zirconium does not have a nature to form a solid solution where the oxide layer becomes degraded by long use, it may be all removed by sand blasting and there can be again produced a fresh oxide layer having the same type as the preceding one by heat-treatment in an oxidizing atmosphere.

As mentioned above, an oxide layer deposited on the surface of ferroalloy articles by the method of the present invention mainly consists of aAl O as a main component and oxides of titanium, zirconium and other elements mixed therewith. Said oxide layer is really of quite a novel type in the sense that it is formed compact and firm on the surface of a metallic base body and displays a very strong resistance to erosion and peeling even where it is repeatedly heated by corrosive gases containing halogens of compounds thereof which are introduced at high temperature and speed, and moreover well withstands general oxidation. The method of the present invention is adapted for use not only in a reaction vessel for treating exhaust gases from automobiles, but also widely in metallic articles which are exposed to general severely corrosive gases containing halogens or compounds thereof with a high temperature.

EXAMPLES General The invention is more clearly set forth in the following examples. The compositions of ferroalloys constituting a base KING body used in these examples and the ratio of (Ti% Zr%)/N% associated therewith are collectively presented in Table 3 below.

The oxide layers of Examples 1 to 4 are all of the aforementioned Type A, while those of Examples 5 to 16 are all of the above-described Type B. FIG. 7 is a co-ordinate system where the titanium content is plotted on the ordinate and the zirconium content on the abscissa. As seen from this figure, if, with 0.2 percent taken as the base point, the zirconium content decreases therefrom, there will result Type A of oxide layer. Where the zirconium content rises above the above-mentioned base point of 0.2 percent, the resultant oxide layer will TABLE 3. COMPOSITIONS OF SAMPLE METALS Example EXAMPLE I An alloy melted in the atmosphere was rolled into a thin plate. Samples cut out of the plate were divided into three groups. The samples were heated in an open small electric furnace at l,200 C for 2, l0 and 50 hours for the respective groups. After said heat-treatment, the samples were allowed to cool in the air. On their surfaces were formed oxide layers of Type A varying in thickness for each group. The samples were subjected to an erosion test for 2 hours at 900 C by the aforementioned method. None of the samples presented any erosion at all, namely, they all displayed an excellent resistance to erosion and oxidation, though they indicated 100 percent in the previously defined rate of peeling. The thickness of an oxide layer formed averaged 4.5 microns for 2 hours of heating, microns for 10 hours of heating and microns for 50 hours of heating.

With those of the aforesaid samples which were heated 50 hours, the oxide layer formed on the surface thereof was completely removed by sand blasting. When said samples were heated again 50 hours at 1,300 C, there was generated the same type of oxide layer as described above, enabling them fully to withstand the same test as initially conducted.

EXAMPLE 2 Samples obtained in the similar manner as in Example I were divided into three groups as in Example 1 by the different numbers of hours used in heating them at l,300 C. On the surfaces of the samples were deposited oxide layers of Type A varying in thickness for each group. Said thickness averaged 3.5 microns for 2 hours of heating, 8 microns for 10 hours of heating and 15 microns for 50 hours of heating. The samples presented the same resistance to erosion and oxidation and rate of peeling as in Example 1.

EXAMPLE 3 Samples obtained in the same manner as in Example I were divided into five groups by different lengths of time, that is, I0, 20, 50, I00 and 200 hours used in heating them at L300 C. On the surfaces of the samples were formed oxide layers of Type A varying in thickness of each group. Said thickness averaged 8, 11, l5, l7 and 19 microns for the respective groups. The relationship of the numbers of hours used in heating each group and the weight increase due to oxidation is presented in FIG. 6.

The thin metal plate prepared according to Example 3 was worked into an after burner type reaction vessel for treating exhaust gases from automobiles. The vessel was heated 20 hours in an electric furnace kept at a temperature of l,300 C to form inside and outside thereof an oxide layer of Type A about l0 microns thick. An after burner in which there was incorporated the reaction vessel was fitted to a 1,500 c.c. engine. After a 500 hours continuous run of the engine at 5,000 r.p.m., the exhaust gas therefrom was burnt again. This bench test corresponded to the actual run of an automobile exceeding 50,000 kilometers. The test proved that the reaction vessel was not attacked at all by the exhaust gas, nor presented cracks, giving very satisfactory results from a practical point of view.

EXAMPLE 4 A sample obtained in the same manner as in Example I was heated 20 hours at I,300 C. An oxide layer formed thereon was ofType A and had a thickness of l l microns.

EXAMPLE 5 An oxide layer deposited on the surface of a sample treated as in Example 4 had a structure intermediate between Types A and B and was 30 microns thick. The layer was relatively deeply settled in the metallic base body, so that its rate of peeling was reduced to 48 percent.

EXAMPLE 6 Samples obtained in the same manner as in Example 3 were heated under the same condition as in said example to form oxide layers of Type B varying in thickness for each group of samples. The thickness averaged 10 microns for 2 hours of heating, 22 microns for 10 hours of heating and 35 microns for 50 hours of heating. An erosion test conducted 2 hours at 900 C showed that all the samples displayed an excellent resistance to erosion and oxidation, the rate of peeling being around 30 percent.

EXAMPLE 7 Samples prepared in the same manner as in Example I were divided into three groups which were heated 2, l0 and 50 hours respectively at l,300 C. On the surfaces of the samples were formed oxide layers of Type B varying in thickness for each group. The thickness averaged 19 microns for 2 hours of heating, 43 microns for 10 hours of heating and 99 microns for 50 hours of heating. An erosion test conducted 2 hours at l,l00 C showed that all the samples displayed an excellent re sistance to erosion and oxidation, the rate of peeling being around 30 percent.

EXAMPLE 8 Samples obtained in the same manner as in Example I were divided into three groups which were heated 20, 50 and I00 hours respectively at l,300 C to form on the surfaces of the samples oxide layers of Type B varying in thickness for each group. The thickness averaged 58 microns for 20 hours of heating, 1 10 microns for 50 hours of heating and 150 microns for 100 hours of heating.

A thin metal plate prepared according to this example was worked into an after burner type reaction vessel for treating exhaust gas from an automobile. The vessel was heated 20 hours in an electric furnace kept at l,300 C to form inside and outside of the vessel an oxide layer of Type B about 50 to 60 microns thick. An after-burner in which the vessel was incorporated was fitted to an automobile equipped with a Wankel type rotary piston engine of 500 c.c. X 2 rotor type. After the automobile was actually made to run 50,000 kilometers, the after burner was checked, showing that the inner wall of the reaction vessel was not attacked at all by the-exhaust gas, nor presented a creep phenomenon due to a continuous run, giving very desirable results from a practical standpoint.

EXAMPLES 9 to 16 Throughout these examples, samples prepared in the same manner as in Example 1 were heated 20 hours at l,300 C. The resultant oxide layers were all of Type B, the thickness averaging 34, 42, 26, 73, 39, 30, 33 and 83 microns in the order of Examples 9 to 16. The oxide layers also displayed the rates of peeling as 32, 32, 48, 21, 22, 42, 30 and 23 percent in the same order.

What we claim is:

l. A method of preparing corrosion-resistant metallic arti-v cles which comprises forming articles from a metal consisting of 10 to 25 percent of chromium, 2 to percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.1 to 0.6 percent of titanium, 0.01 to 0.5 percent of zirconium, iron as the remainder and. further containing ordinary impurities, in which constitution ratio of Ti% Zr% to N% ranges from to 60, heating the article in an oxidizing atmosphere for a suitable number of hours at a temperature of from l,000 to l,400 C to diffuse by migration in the surface layer of the base body of the article part of oxidizable elements contained therein, thereby forming on said surface an oxide layer mainly consisting of aAl O 3 to 300 microns thick.

2. A method of preparing corrosi0n-resistant metallic articles which comprises forming articles from a metal consisting of 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.1 to 0.6 percent of titanium; 001 to 0.2 percent of zirconium, iron as the remainder and further containing ordi nary impurities, in which constitution ratio of Ti% Zr% to N% ranges from 10 to 60, heating the article in an oxidizing atmosphere for a suitable number of hours at a temperature of from 1,000 to 1,400 C to diffuse by migration in the surface layer of the base body of the article part of oxidizable elements contained therein, thereby forming on said surface an oxide layer averaging from 3 to I00 microns in thickness which mainly consists of 1,0,, and the outermost region of which is constituted by a thin film of Al O -TiO 3. A method of preparing corrosion-resistant metallic articles which comprises forming articles from a metal consisting of 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.2 to 0.6 percent of titanium, more than 0.2 to 0.5 percent of zirconium, iron as the remainder and further containing ordinary impurities, in which constitution ratio of Ti% Zr% to N ranges from 10 to 60 and the ratio of Ti% to Zr% is set at a larger value than 0.9, heating the article in an oxidizing atmosphere for a suitable number of hours at a temperature of from l,000 to l,400 C to diffuse by migration in the surface layer of the base body of the article part of oxidizable elements contained therein, thereby forming on said surface an oxide layer 3 to 300 microns thick on average mainly consisting of 04.41 0 settled in the metallic base body in a rooty form.

4. Corrosion-resistant metallic article comprising 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 01 to 0.6 percent of titanium, 0.01 to 0.5 percent of zirconium, iron as the remainder and further containing ordinary impurities, in which the constitution ratio of Ti Zr to N ranges from 10 to 60, and having a surface oxide layer consisting essentially of aAl O 3 to 300 microns thick.

5. Article of claim 4, wherein zirconium is from 0.01 to 0.2 percent, and the oxide layer has a thickness of 3 to microns and the outermost region of which is a thin film of Al O -TiO 6. Article of claim 4, wherein titanium is from 0.2 to 0.6 percent, zirconium is from 0.2 to 0.5 percent, the weight ratio of titanium to zirconium is greater than 0.9, and said oxide layer is settled in the article in a rooty form. 

2. A method of preparing corrosion-resistant metallic articles which comprises forming articles from a metal consisting of 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.1 to 0.6 percent of titanium, 0.01 to 0.2 percent of zirconium, iron as the remainder and further containing ordinary impurities, in which cOnstitution ratio of Ti% + Zr% to N% ranges from 10 to 60, heating the article in an oxidizing atmosphere for a suitable number of hours at a temperature of from 1,000* to 1,400* C to diffuse by migration in the surface layer of the base body of the article part of oxidizable elements contained therein, thereby forming on said surface an oxide layer averaging from 3 to 100 microns in thickness which mainly consists of Alpha Al2O3, and the outermost region of which is constituted by a thin film of Al2O3.TiO2.
 3. A method of preparing corrosion-resistant metallic articles which comprises forming articles from a metal consisting of 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.2 to 0.6 percent of titanium, more than 0.2 to 0.5 percent of zirconium, iron as the remainder and further containing ordinary impurities, in which constitution ratio of Ti% + Zr% to N% ranges from 10 to 60 and the ratio of Ti% to Zr% is set at a larger value than 0.9, heating the article in an oxidizing atmosphere for a suitable number of hours at a temperature of from 1,000* to 1,400* C to diffuse by migration in the surface layer of the base body of the article part of oxidizable elements contained therein, thereby forming on said surface an oxide layer 3 to 300 microns thick on average mainly consisting of Alpha Al2O3 settled in the metallic base body in a rooty form.
 4. Corrosion-resistant metallic article comprising 10 to 25 percent of chromium, 2 to 5 percent of aluminum, less than 0.04 percent of carbon, 0.005 to 0.05 percent of nitrogen, 0.1 to 0.6 percent of titanium, 0.01 to 0.5 percent of zirconium, iron as the remainder and further containing ordinary impurities, in which the constitution ratio of Ti % + Zr % to N % ranges from 10 to 60, and having a surface oxide layer consisting essentially of Alpha Al2O3 3 to 300 microns thick.
 5. Article of claim 4, wherein zirconium is from 0.01 to 0.2 percent, and the oxide layer has a thickness of 3 to 100 microns and the outermost region of which is a thin film of Al2O3.TiO2
 6. Article of claim 4, wherein titanium is from 0.2 to 0.6 percent, zirconium is from 0.2 to 0.5 percent, the weight ratio of titanium to zirconium is greater than 0.9, and said oxide layer is settled in the article in a rooty form. 